Weld Inspection & Acceptance Criteria

Welding inspection is the process of checking welds to ensure they meet specified quality requirements. Inspection may include visual examination, dimensional checks and non-destructive testing (NDT). Inspection helps identify defects before components are placed into service.

Visual inspection is the first stage of weld quality assessment. The inspector checks weld size, profile, penetration, alignment and surface defects such as cracks, undercut or excessive spatter. Many welding defects can be identified quickly using visual inspection alone.

Non-Destructive Testing (NDT) is a group of inspection methods used to examine welds without damaging the component. Common NDT methods include ultrasonic testing, radiographic testing, magnetic particle inspection and dye penetrant testing.

Ultrasonic Testing uses high-frequency sound waves to detect internal weld defects such as lack of fusion, slag inclusions and cracks. UT is widely used for thick welds where internal quality must be verified without cutting the component.

Radiographic Testing uses X-rays or gamma rays to produce an image of the internal structure of a weld. RT is particularly effective at identifying porosity, slag inclusions and internal defects that cannot be seen visually.

Dye Penetrant Testing involves applying a coloured or fluorescent liquid to the surface of a weld. The dye enters surface-breaking defects, making cracks and other discontinuities visible after developer is applied. PT is commonly used on stainless steel and non-magnetic materials.

Magnetic Particle Inspection detects surface and near-surface cracks in ferromagnetic materials by applying a magnetic field and fine magnetic particles. It is commonly used on carbon steel welds but cannot be used on non-magnetic materials such as aluminium or austenitic stainless steel.

The timing of NDT depends on the welding procedure, material and project requirements. Inspection may be carried out during fabrication, after welding or following heat treatment. The required inspection method should be specified in the project documentation or welding procedure.

ISO 5817 specifies quality levels for imperfections in welded joints. It defines acceptance criteria for weld defects based on three quality levels: B, C and D. The required quality level depends on the application and customer specification.

Acceptance criteria define the maximum allowable size and type of weld imperfections. They are specified by standards, customer requirements or project specifications. Welds that exceed the acceptance criteria may require repair or replacement.

A weld defect is an imperfection that reduces the quality or performance of a welded joint. Common defects include porosity, cracks, undercut, lack of fusion and slag inclusions. Identifying defects early helps reduce repair costs and improve product quality.

Common welding defects include porosity, lack of fusion, incomplete penetration, undercut, slag inclusions, overlap, excessive reinforcement and weld cracking. Most defects can be prevented through correct preparation, machine settings and consumable selection. For prevention and fixes, see our welding defects and troubleshooting guides.

The right method depends on what you need to find and on the material. Visual inspection comes first and catches many surface defects. Surface-breaking cracks are found with dye penetrant testing (suitable for stainless and non-magnetic materials) or magnetic particle inspection (for ferromagnetic materials such as carbon steel, but not aluminium or austenitic stainless steel). Internal defects in thicker sections are found with ultrasonic or radiographic testing. The welding procedure or project documentation should specify which methods apply, at what stage, and the acceptance criteria — often an ISO 5817 quality level — that the welds must meet.